Weak First-order Phase Transition Research Articles

Bubble nucleation in the two-flavor quark-meson model* *Supported in part by the National Natural Science Foundation of China (NSFC) (11675048)

We investigate the dynamics of a first-order quark-hadron transition via homogeneous thermal nucleation in the two-flavor quark-meson model. The contribution of the fermionic vacuum loop in the effective thermodynamics potential and phase diagram, together with the location of the critical endpoint (CEP), is obtained in the temperature and chemical potential plane. For weak and strong first-order phase transitions, by taking the temperature as a variable, the critical bubble profiles, evolutions of the surface tension, and saddle-point action in the presence of a nucleation bubble are numerically calculated in detail when fixing the chemical potentials at and . Our results show that the system could be trapped in the metastable state for a long time as long as the temperature is between the metastable region characterized by the up and low spinodal lines. Moreover, the surface tension at criticality will rise to approximately when the chemical potential is very high. Such a small surface tension value would favor a mixed phase in the cores of compact stars and may have an important implication in astrophysics.

Constraints on Phase Transitions in Neutron Star Matter

Recent inference results of the sound velocity in the cores of neutron stars are summarized. Implications for the equation of state and the phase structure of highly compressed baryonic matter are discussed. In view of the strong constraints imposed by the heaviest known pulsars, the equation of state must be very stiff in order to ensure the stability of these extreme objects. This required stiffness limits the possible appearance of phase transitions in neutron star cores. For example, a Bayes factor analysis quantifies strong evidence for squared sound velocities cs2>0.1 in the cores of 2.1 solar-mass and lighter neutron stars. Only weak first-order phase transitions with a small phase coexistence density range Δρ/ρ<0.2 (at the 68% level) in a Maxwell construction still turn out to be possible within neutron stars. The central baryon densities in even the heaviest neutron stars do not exceed five times the density of normal nuclear matter. In view of these data-based constraints, much discussed issues such as the quest for a phase transition towards restored chiral symmetry and the active degrees of freedom in cold and dense baryonic matter, are reexamined.

Evidence for a Phase Transition in the Quantum Spin Liquid State of a Kitaev Candidate α-RuCl3

The Kitaev quantum spin liquid (QSL) on the two-dimensional honeycomb lattice epitomizes an entangled topological state, where the spins fractionalize into Majorana fermions. This state has aroused tremendous interest because it harbors non-Abelian anyon excitations. The half-integer quantized thermal Hall (HIQTH) conductance observed in $\alpha$-RuCl$_3$ is a key signature of these excitations. However, the fate of this topologically nontrivial state at intense fields remains largely elusive. Here, we report the thermal conductivity $\kappa$ and specific heat $C$ of $\alpha$-RuCl$_3$ with in-plane magnetic fields $H$. For the field direction perpendicular to the Ru-Ru bond, where the HIQTH effect is observed, we find a discontinuous jump in $\kappa(H)$ and a peak anomaly in $C(H)$ at $\mu_0H^*\approx11$\,T, evidencing a weak first-order phase transition. Remarkably, the HIQTH effect vanishes close to $H^*$. Furthermore, we find that the spin-fractionalization feature is retained well above $H^\ast$. These imply the emergence of the phase transition that separates two QSL phases with distinct topological properties.

Lattice instability, anharmonicity and Raman spectra of BaO under high pressure: A first principles study

Alkaline-earth metal oxides, MgO and CaO dominate Earth's lower mantle, therefore, exploring high-pressure behavior of these compounds is of significant geophysical research interest. Among the alkaline-earth metal oxides, BaO exhibits rich polymorphism in the pressure range of 0–1.5 Mbar. Static enthalpy calculations revealed that BaO undergoes a pressure-induced structural phase transition from NaCl-type (B1) → NiAs-type (B8) → distorted CsCl-type (d-B2) → CsCl-type (B2) at 5.1, 19.5 and 120 GPa, respectively. Transitions B1 → B8 and B8 → d-B2 are found to be first order in nature whereas d-B2 → B2 is a second order or weak first-order phase transition with displacive nature. Interestingly, the d-B2 phase shows stability over a wide pressure range of ∼19.5–113 GPa. Mechanical and dynamical stabilities of ambient and high-pressure phases are demonstrated through the computed second-order elastic constants and phonon dispersion curves, respectively. Under high pressure, significant phonon softening and soft phonon mode along the M-direction are observed for B8, d-B2 and B2 phases, respectively. Pressure-dependent Raman spectra suggest a phase transition from d-B2 to a Raman inactive phase under high pressure. Overall, the present study provides a comprehensive understanding of lattice dynamics and the underlying mechanisms behind pressure-induced structural phase transitions in BaO.

Nematic confined phases in the U(1) quantum link model on a triangular lattice: Near-term quantum computations of string dynamics on a chip

The $\text{U}(1)$ quantum link model on the triangular lattice has two rotation-symmetry-breaking nematic confined phases. Static external charges are connected by confining strings consisting of individual strands with fractionalized electric flux. The two phases are separated by a weak first-order phase transition with an emergent, almost exact $\text{SO}(2)$ symmetry. We construct a quantum circuit on a chip to facilitate near-term quantum computations of the nontrivial string dynamics.

Experimental and Theoretical Investigations of Fe-Doped Hexagonal MnNiGe.

We report a comprehensive investigation of MnNi0.7Fe0.3Ge Heusler alloy to explore its magnetic, caloric, and electrical transport properties. The alloy undergoes a ferromagnetic transition across TC ∼ 212 K and a weak-antiferromagnetic transition across Tt ∼ 180 K followed by a spin-glass transition below Tf ∼ 51.85 K. A second-order phase transition across TC with mixed short and long-range magnetic interactions is confirmed through the critical exponent study and universal scaling of magnetic entropy and magnetoresistance. A weak first-order phase transition is evident across Tt from magnetization and specific heat data. The frequency dependent cusp in χAC(T) along with the absence of a clear magnetic transition in specific heat C(T) and resistivity ρ(T) establish the spin glass behavior below Tf. Mixed ferromagnetic and antiferromagnetic interactions with dominant ferromagnetic coupling, as revealed by density functional calculations, are experimentally evident from the large positive Weiss temperature, magnetic saturation, and negative magnetic-entropy and magnetoresistance.

Synthesis, structure and transport properties of high-pressure modification VO2(S)

A new high pressure modification VO2(S) was obtained by the thermobaric treatment of stoichiometric mixture of powders V2O3+V2O5 at Р = 2.0–7.0 GPa and Т = 600–900°С. The crystal structure of VO2(S) has been studied using X-ray powder diffraction: a = 7.35367(3), c = 4.51507(4) Å, V = 211.448(1) Å3 at T = 296(1) K, Z = 7, space group P3. The V atoms are six-coordinated within distorted VO6 octahedra. The structure is double-layered and built up of triple units of edge-shared octahedra. The connection of these units forms one solid layer, while the second one contains separate units. Structural relationships between VO2(S), VO2(M2) and WO2-hp are discussed. Electric and X-ray diffraction measurements show that VO2(S) undergoes a weak first-order phase transition at temperature around 290 K. We also carried out density functional calculations of VO2(S) using the hybrid HSE06 exchange-correlation functional. The charge and heat transport properties are evaluated within the semi-classical Boltzmann approach.

Weakly first-order quantum phase transition between spin-nematic and valence-bond crystal order in a square-lattice SU(4) fermionic model

We consider a model Hamiltonian with two $\mathrm{SU}(4)$ fermions per site on a square lattice, showing a competition between bilinear and biquadratic interactions. This model has generated interest due to possible realizations in ultra-cold-atom experiments and existence of spin-liquid ground states. Using a basis transformation, we show that part of the phase diagram is amenable to quantum Monte Carlo simulations without a sign problem. We find evidence for spin nematic and valence-bond crystalline phases, which are separated by a weak first-order phase transition. A $\mathrm{U}(1$) symmetry is found to emerge in the valence-bond crystal histograms, suggesting proximity to a deconfined quantum critical point. Our results are obtained with the help of a loop algorithm which allows large-scale simulations of bilinear-biquadratic $\mathrm{SO}(N$) models on arbitrary lattices in a certain parameter regime.

Probing three-state Potts nematic fluctuations by ultrasound attenuation

Motivated by recent studies of three-state Potts nematic states in magic-angle twisted bilayer graphene and doped-${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, we analyze the impact of critical nematic fluctuations on the low-energy properties of phonons. In this study, we propose how to identify the three-state Potts nematic fluctuations by ultrasound attenuation. The Gaussian fluctuation analysis shows that the Landau damping term becomes isotropic due to fluctuations of the ${C}_{3}$-breaking bond-order, and the nematoelastic coupling is also shown to be isotropic. These two features lead to an isotropic divergence of the transverse sound attenuation coefficient and an isotropic lattice softening, in contrast to the case of the ${C}_{4}$-breaking bond-order, which shows strong anisotropy. Moreover, we use a mean-field approximation and discuss the impurity effects. The transition temperature takes its maximum near the filling of the van Hove singularity, and the large density of states favors the nematic phase transition. It turns out that the phase transition is of weak first-order in the wide range of filling and, upon increasing the impurity scattering, the first-order transition line at low temperatures gradually shifts towards the second-order line, rendering the transition a weak first-order in a wider range of parameters. Furthermore, it is confirmed that the enhancement of the ultrasound attenuation coefficient will be clearly observed in experiments in the case of a weak first-order phase transition.

Phase transition of four-dimensional lattice ϕ4 theory with tensor renormalization group

We investigate the phase transition of the four-dimensional single-component ${\ensuremath{\phi}}^{4}$ theory on the lattice using the tensor renormalization group method. We have examined the hopping parameter dependence of the bond energy and the vacuum condensation of the scalar field $⟨\ensuremath{\phi}⟩$ at a finite quartic coupling $\ensuremath{\lambda}$ on large volumes up to $V=102{4}^{4}$ in order to detect the spontaneous breaking of the ${\mathbb{Z}}_{2}$ symmetry. Our results show that the system undergoes the weak first-order phase transition at a certain critical value of the hopping parameter. We also make a comparative study of the three-dimensional ${\ensuremath{\phi}}^{4}$ theory and find that the properties of the phase transition are consistent with the universality class of the three-dimensional Ising model.

Extensive studies of the neutron star equation of state from the deep learning inference with the observational data augmentation

We discuss deep learning inference for the neutron star equation of state (EoS) using the real observational data of the mass and the radius. We make a quantitative comparison between the conventional polynomial regression and the neural network approach for the EoS parametrization. For our deep learning method to incorporate uncertainties in observation, we augment the training data with noise fluctuations corresponding to observational uncertainties. Deduced EoSs can accommodate a weak first-order phase transition, and we make a histogram for likely first-order regions. We also find that our observational data augmentation has a byproduct to tame the overfitting behavior. To check the performance improved by the data augmentation, we set up a toy model as the simplest inference problem to recover a double-peaked function and monitor the validation loss. We conclude that the data augmentation could be a useful technique to evade the overfitting without tuning the neural network architecture such as inserting the dropout.

Bubble dynamics in a strong first-order quark-hadron transition * *Supported in part by National Natural Science Foundation of China (NSFC) (11675048)

We investigate the dynamics of a strong first-order quark-hadron transition driven by cubic interactions via homogeneous bubble nucleation in the Friedberg-Lee model. The one-loop effective thermodynamic potential of the model and the critical bubble profiles have been calculated at different temperatures and chemical potentials. By taking the temperature and the chemical potential as variables, the evolutions of the surface tension, the typical radius of the critical bubble, and the shift in the coarse-grained free energy in the presence of a nucleation bubble are obtained, and the limit on the reliability of the thin-wall approximation is also addressed accordingly. Our results are compared to those obtained for a weak first-order quark-hadron phase transition; in particular, the spinodal decomposition is relevant.

Phase transitions of two spin-1/2 Baxter–Wu layers coupled with Ising-type interactions

Using a Monte Carlo simulation and the single histogram reweighting technique, we study the critical behaviors and phase transitions of the Baxter–Wu (BW) model on a two-layer triangular lattice with Ising-type interlayer couplings. Via the finite-size analysis, we obtain the transition temperatures and critical exponents at repulsive and attractive interlayer couplings. The data for the repulsive interlayer coupling suggest continuous transitions, and the critical behaviors are the same as those of the 2D BW model, belonging to the four-state Potts universality class. The reduced energy cumulants and the histograms reveal that attractive coupling leads to weak first-order phase transitions. The pseudocritical exponents with the existence of the interlayer couplings indicate that the first-order transition is very close to the critical point of the 2D standard BW model.

Phase diagram for the bisected-hexagonal-lattice five-state Potts antiferromagnet.

In this paper we study the phase diagram of the five-state Potts antiferromagnet on the bisected-hexagonal lattice. This question is important since Delfino and Tartaglia recently showed that a second-order transition in a five-state Potts antiferromagnet is allowed, and the bisected-hexagonal lattice had emerged as a candidate for such a transition on numerical grounds. By using high-precision Monte Carlo simulations and two complementary analysis methods, we conclude that there is a finite-temperature first-order transition point. This one separates a paramagnetic high-temperature phase, and a low-temperature phase where five phases coexist. This phase transition is very weak in the sense that its latent heat (per edge) is two orders of magnitude smaller than that of other well-known weak first-order phase transitions.

The mass scales of the Higgs field

In the first version of the theory, with a classical scalar potential, the sector inducing SSB was distinct from the Higgs field interactions induced through its gauge and Yukawa couplings. We have adopted a similar perspective but, following most recent lattice simulations, described SSB in [Formula: see text] theory as a weak first-order phase transition. In this case, the resulting effective potential has two mass scales: (i) a lower mass [Formula: see text], defined by its quadratic shape at the minima, and (ii) a larger mass [Formula: see text], defined by the zero-point energy. These refer to different momentum scales in the propagator and are related by [Formula: see text], where [Formula: see text] is the ultraviolet cutoff of the scalar sector. We have checked this two-scale structure with lattice simulations of the propagator and of the susceptibility in the 4D Ising limit of the theory. These indicate that, in a cutoff theory where both [Formula: see text] and [Formula: see text] are finite, by increasing the energy, there could be a transition from a relatively low value, e.g. [Formula: see text] GeV, to a much larger [Formula: see text]. The same lattice data give a final estimate [Formula: see text] GeV which induces to reconsider the experimental situation at Large Hadron Collider (LHC). In particular an independent analysis of the ATLAS[Formula: see text]+[Formula: see text]CMS data indicating an excess in the 4-lepton channel as if there were a new scalar resonance around 700 GeV. Finally, the presence of two vastly different mass scales, requiring an interpolating form for the Higgs field propagator also in loop corrections, could reduce the discrepancy with those precise measurements which still favor large values of the Higgs particle mass.

Finite temperature effects in quantum systems with competing scalar orders

The study of the competition or coexistence of different ground states in many-body systems is an exciting and actual topic of research, both experimentally and theoretically. Quantum fluctuations of a given phase can suppress or enhance another phase depending on the nature of the coupling between the order parameters, their dynamics and the dimensionality of the system. The zero temperature phase diagrams of systems with competing scalar order parameters with quartic and bilinear coupling terms have been previously studied for the cases of a zero temperature bicritical point and of coexisting orders. In this work, we apply the Matsubara summation technique from finite temperature quantum field theory to introduce the effects of thermal fluctuations on the effective potential of these systems. This is essential to make contact with experiments. We consider two and three-dimensional materials characterized by a Lorentz invariant quantum critical theory, i.e., with dynamic critical exponent z = 1, such that time and space scale in the same way. We obtain that in both cases, thermal fluctuations lead to weak first-order temperature phase transitions, at which coexisting phases arising from quantum corrections become unstable. We show that above this critical temperature (Tc), the system presents scaling behavior consistent with that approaching a quantum critical point. Below the transition the specific heat has a thermally activated contribution with a gap related to the size of the domains of the ordered phases. We obtain that Tc decreases as a function of the distance to the zero temperature classical bicritical point (ZTCBP) in the coexistence region, implying that in our approach, the system attains the highest Tc above the fine tuned value of this ZTCBP.

Conformality Loss, Walking, and 4D Complex Conformal Field Theories at Weak Coupling.

Four-dimensional gauge theories with matter can have regions in parameter space, often dubbed conformal windows, where they flow in the infrared to nontrivial conformal field theories. It has been conjectured that conformality can be lost because of merging of two nearby fixed points that move into the complex plane, and that a walking dynamics governed by scaling dimensions of operators defined at such complex fixed points can occur. We find controlled, parametrically weakly coupled, and ultraviolet-complete 4D gauge theories that explicitly realize this scenario. We show how the walking dynamics is controlled by the coupling of a double-trace operator that crosses marginality. The walking regime ends when the renormalization group flow of this coupling leads to a (weak) first-order phase transition with Coleman-Weinberg symmetry breaking. A light dilatonlike scalar particle appears in the spectrum, but it is not parametrically lighter than the other excitations.

Phase transition of four-dimensional Ising model with higher-order tensor renormalization group

We apply the higher-order tensor renormalization group (HOTRG) to the four-dimensional ferromagnetic Ising model, which has been attracting interests in the context of the triviality of the scalar $\phi^4_{d=4}$ theory. We investigate the phase transition of this model with HOTRG enlarging the lattice size up to $1024^4$ with parallel computation. The results for the internal energy and the magnetization are consistent with the weak first-order phase transition.

Physical Properties of Ca-Doped Double Perovskite La2NiMnO6

Samples of double perovskite manganese La2−xCaxNiMnO6 (x = 0, 0.05, 0.15, 0.2) are prepared by a traditional solid-state reaction. X-ray diffraction patterns show that four samples all exhibit a good uniform single phase and have perovskite crystal structures. From the temperature dependence of the magnetization in a 5 mT magnetic field (M–T) and the field dependence of the magnetization near the Curie temperature (M–H) for La2−xCaxNiMnO6 (x = 0, 0.05, 0.15, 0.2), the ferromagnetism of the samples is weakened, while the antiferromagnetism is enhanced with Ca doping. Hysteresis loops at 2 K, enlarged views of the low-field hysteresis loops at 2 K under a field cooling process with a cooling field of 1T and Raman spectra at room temperature for La2−xCaxNiMnO6 (x = 0, 0.05, 0.15, 0.2) show that when the Ca doping amount is x = 0.05, compared with La2NiMnO6, the antisite disorder degree of the system is almost the same, but the number of antiphase boundaries increases. With further Ca doping, the antisite disorders and numbers of antisite defects and antiferromagnetic antiphase boundaries increase, simultaneously strengthening the enhancement in the antiferromagnetic coupling. In addition, as the Ca doping increases, the upward deviation of the temperature-dependent χ−1 ~ T for bulk La2−xCaxNiMnO6 (LCNMO) from the Curie–Weiss law becomes more obvious, which shows that Ca doping enhances the long-range ferromagnetic order, antiferromagnetic clusters increase, and long-range ferromagnetic order dominates the magnetic behaviour near the ferromagnetic transition point. Finally, we discuss the magnetic entropy and electrical changes of four samples and confirm that Ca doping causes the systems to undergo a weak first-order phase transition.

Liquid crystal blue phases interconversions based real-time thermal imaging device.

The real-time thermal imaging systems, which allow the rapid conversion, acquisition, and manipulation of obtained optical information, are the emerging technologies that offer a variety of imaging applications. Here, we present a novel type of thermal imaging device, based on the thermo-optical properties of liquid crystal blue phases. Herewith, the novelty lies in the use of a weak first-order phase transition between the blue phases controlled by external thermal fields. In turn, the stimulated interconversions of the selective reflections between the blue phases enable the visualization of the two-dimensional spatial distribution of the thermal fields. The real-time room temperature operation capabilities of the proposed thermal imaging device may enable applications in areas such as medicine, astronomy, security, surveillance, people tracking, aerospace monitoring, and artworks inspection.